1. Field of the Invention
The invention relates to a method for producing a flat steel product which can be readily formed and which has a C content of from 0.1 to 0.4% by weight, wherein the flat steel product is subjected to an annealing treatment in a continuous furnace.
The invention further relates to a correspondingly produced flat steel product and a method for producing components from such a flat steel product.
2. Background of Related Art
Flat steel products of the type in question in this instance are required in particular for the production of bodywork and chassis components for motor vehicles. In this instance, extremely high demands are placed on the flat steel products in terms of their forming properties. This relates to both cold and hot-formability.
The hot forming of galvanised flat steel products to form high-strength or extremely high-strength steel components is particularly problematic. With such steel components, the protective coating which is generally based on zinc or a zinc alloy provides sufficient cathodic corrosion protection.
However, if a steel sheet which is provided with a metal corrosion protection coating, for the hot forming and hardening operation which may be carried out subsequently or in combination with the hot forming, must be heated to a temperature which is above the melting temperature of the metal of the protective coating, there is a danger of so-called “liquid metal embrittlement”. This embrittlement of the steel occurs when molten metal of the coating enters the notches which are formed on the surface of the respective flat steel product during forming. The liquid metal which reaches the steel substrate is precipitated there at the grain boundaries and thus reduces the maximum tensile and compressive stresses which can be taken up.
The danger of liquid metal embrittlement with higher-strength and high-strength steels which have only limited ductility and which consequently have a tendency to form cracks near the surface when they are formed has been found to be particularly critical.
From JP 60-159120 A it is generally known that the bending properties of a steel sheet can be improved by means of a decarbonisation treatment, by means of which there is produced an edge layer which is close to the surface, is from 20 to 100 μm thick and has a reduced C content with respect to the core region of the steel sheet. However, this measure in this prior art has no connection to steel sheets which are covered with a metal protective coating, nor does it relate to higher-strength steels or high-strength steels with C contents of at least 0.1% by weight.
The tendency for decarbonisation of a carbon-containing steel alloy results from the oxidation behaviour of the carbon released. Owing to its great movability, the carbon which is released in the lattice has a tendency towards effusion during a heat treatment. The decarbonisation which takes place depending on the C-potential of the gas phase, under which the heat treatment takes place, with or without simultaneous scaling, therefore represents one of the oldest problems in the production and processing of steel.
In principle, a decarbonisation is carried out in accordance with the Boudouard equilibrium reactions in accordance with the following reaction processes:[C]+½O2<->CO[C]+O2<->CO2 [C]+CO2<->2CO[C]+H2<->CH4 where [C]=carbon released
In commercial annealing installations having a typical protective gas atmosphere which contains hydrogen, nitrogen and water vapour, the following equilibrium reaction is produced:H2+½O2<->H2O
Gas atmospheres containing water have been found to be particularly reactive with respect to carbon. Therefore, an additional heterogeneous equilibrium reaction which is particularly significant in practice supplements the decarbonisation reactions mentioned:[C]+H2O<->CO+H2 
When used in a selective manner, properties of a steel product determined by means of a decarbonisation can be improved.
In order to be able to use this knowledge effectively in practice, the “Open Coil” method is proposed in GB 1 189 464. In this method, a hard-rolled, cold-rolled strip is wound so loosely to form a coil that a free space is provided in each case between the individual winding layers of the coil. The annealing gas which flows through the free spaces during the subsequent annealing treatment in a hood type furnace then flows over the entire steel surface in a uniform manner so that a uniform decarbonisation result is achieved over the entire length of the processed steel strip. However, the annealing treatment which is carried out in this manner takes several hours.
A method which can be carried out more economically for decarbonisation annealing of steel strip in a continuous furnace under a reducing annealing atmosphere is described in DE-OS 2 105 218. According to this known method, the respective steel strip is annealed at an annealing temperature of less than 780° C. over a sufficiently long annealing period until the carbon content in the steel strip is less than 0.01% when leaving the continuous furnace. Subsequently, the steel strip can be provided with a hot-dip coating in order to improve its corrosion resistance. The sheet steel which is produced in this manner has particularly good formability. However, its strength values do not meet the demands which are regularly placed nowadays on flat steel products, from which components for motor vehicle bodywork are intended to be formed.
A suggestion known from WO 2009/024472 A1 also referred to the “Open Coil” method according to which there has been proposed, for a steel strip which comprises a tool steel and which is intended in particular for the production of cutting tools and the like and which has a C content of at least 0.4% by weight, a decarbonisation of the edge layer in order to combine a high level of hardness with good formability. In the region of the decarbonised edge layer, the steel strip which is processed accordingly has increased formability with respect to the base material, whereby the danger of brittle fracture with high external load is reduced.
In contrast to the applications considered in WO 2009/024472 A1, the person skilled in the art, with high-strength and extremely high-strength steels which are intended for the production of high-strength components, generally seeks to avoid, if possible, the decarbonisation or edge decarbonisation which is caused by annealing. It is generally considered that the decarbonisation has a negative influence on the mechanical material properties which are important for these applications.
Following this notion, there has been proposed in DE 102007061489 A1 a method wherein a ductile edge layer which is advantageous for forming is produced on a steel sheet by carrying out selective oxidation of the hardening alloy elements. Any decarbonisation is selectively counteracted in this instance.